The law of the police interview is inviolable – information is only supposed to flow in one direction. But you’ve got to handle grieving parents carefully, otherwise they might write to the Telegraph. Or, in the case of someone like Martin Chorley, call the editor at home.

‘An arrest has been made,’ I said. ‘How it relates to your daughter’s death remains unclear.’

He nodded glumly at this and took a sip of water.

I waited to see if he’d ask who, exactly, had been arrested. When he didn’t, I went back to asking the routine questions that disguised the real reason I was there.

Nightingale’s definition of a rogue practitioner was essentially ‘one that is practising magic without the sanction of the Folly’. Since the only currently sanctioned practitioners were me and him, I’d pointed out that this was not a very useful definition. Besides, there were still a number of wizards of the old school who, despite having ‘rusticated’ themselves, could still practise if they had to. Not to mention all the Rivers, Russian night-witches, fae, demi-fae – and who knew what other kinds of fae – running around doing stuff that looked suspiciously like magic to me.

So we refined our definition down to ‘someone who practised magic in breach of the Queen’s Peace’, and started developing a series of sophisticated tools for determining whether someone’s nearest and dearest might have been dabbling in the metaphysical equivalent of sticking their head in a microwave for fun and profit.

‘Had you noticed any recent changes in Christina’s behaviour?’ I asked. ‘Any sudden new interests?’

‘She’s seventeen,’ he said. ‘So yes, lots of sudden interests.’

He turned his head to look out the window and took a deep breath.

‘Any of them particularly noteworthy?’ I asked.

‘Any of what?’ He turned back to face us.

‘Any of the new interests,’ said Guleed with a note of respectful curiosity – it was her party trick. According to legend she’d once got a confession out of a rapist just by looking sympathetic and nodding occasionally.

‘Me personally,’ Stephanopoulos had said, ‘I’d have nailed his testicles to the chair.’

Ah, the good old days, I’d thought.

Martin Chorley succumbed.

‘History,’ he said. ‘She started reading a great deal of history. I did find it a little bit odd because she wasn’t taking history at A-level.’ He was hazy about exactly when and where her interest had been focused, and I could see that pressing him was just going to make him angry. So I let it go. Tact and fucking diplomacy and all that.

A specialist POLSA team had already turned over Christina Chorley’s room at St Paul’s – I made a note to go over their report and see what she’d had on her shelves.

Martin Chorley said that most of his daughter’s interests had seemed to centre around her phone.